It eventually goes bad but I have used it a couple of months past the expiration date and it worked great. The way I tell if it is still good is to smell of it; also, when it gets too old to use it will get watery. I love to cook with buttermilk.

Do you know you can buy powdered buttermilk? You add the dry powder to the recipe, then add the correct amount of water. The brand is saco cultured (no caps) Buttermilk Blend. I find it in the baking aisle at the grocery. I threw away too many partial quarts of buttermilk, some which had become very chunky. This is a great alternative if you use it only for cooking.